Conventional actuators include columnar actuators and vane-shaped actuators which both utilize air pressure or mechanical spring to cause a shaft to rotate, and further control the shaft to move reciprocatingly to drive a valve body to open or close. Also, a single-action actuator is usually used on a valve body which is usually closed to ensure safety when the actuator is in use.
However, if the conventional single-action actuator uses a spring's resilient force to provide its returning force, the spring has to be installed under a predetermined pressure such that the spring can apply pressure on the shaft even when the spring has been deformed to its returning limit, and if air pressure has to compress the spring, the air pressure has to be increased to a substantially high level. However, the spring may suddenly release pressure to cause the driver of the shaft to violently strike against the actuator and restricting screws, so the actuator structure has to be rigid. Also, the user has to consider several factors of the spring, such as wire diameter, spiral angel, material, thermal treatment, and enhancement of reliability and durability, so it is difficult to design and install the spring.